The influence of ion-implantation on the effective Schottky barrier height of NiGe/n-Ge contacts
Graphical abstract
Comparison of current-voltage characteristics of the Se implantation NiGe/n-Ge contacts with (red) and without (blue) thermal annealing before germanidation. And (b) current-voltage characteristics of NiGe/p-Ge with (red) and without (black) Se implantation.
Introduction
Due to its high carrier mobility, Germanium has attracted growing attention as an alternative to silicon as a channel material for metal-oxidesemiconductor field-effect transistors. But strong Fermi-level pinning in the vicinity of the maximum valence band of Ge raises the effective electron Schottky barrier height (SBH) of metal contacts on n-Ge [1], [2], [3]. Forming quality ohmic contacts with n-Ge remains an obstacle to realizing high performance Ge n-MOSFETs. Several approaches have been proposed to overcome this obstacle, such as inserting interfacial layers (eg. Al2O3, TiO2, Si, ZnO, Ge3N4, TaN et al.) between a metal and n-Ge [4], [5], [6], [7], [8], [9], and implanting and segregating impurities (e.g. P, chalcogen S, Se, and Te) at the metal/n-Ge interface during germanidation [10], [11], [12], [13], [14], [15], [16], [17].
It is well understood that phosphorus segregation at the metal/n-Ge interface increases the tunneling current to achieve a good ohmic contact by narrowing the Schottky barrier. But there is no direct evidence that either S or Se segregation can provide conduction electrons as donors to similarly explain the decrease of the effective SBH of metal/n-Ge contacts [13], [14]. On the other hand, S and Se are considered to play a role in passivating the interface states between the metal and n-Ge, thus alleviating the Fermi-level pinning effect and reducing the barrier height [11], [12], [13].Arguably, the mechanism for modulating electrical properties via Sor Se implantation and segregation is still under debate. In this work, we compare the current-voltage characteristics of NiGe contacts with n-Ge by implantation of P, Se, and Si ions so as to clarify the mechanisms of tuning the effective SBH. The influence of impurity segregation, implantation damage, and interface passivation on the effective SBH, are discussed in detail.
Section snippets
Experimental details
Substrates of NiGe/Ge contacts were created for Sb-doped (2 × 1016 cm−3) n-type Ge (1 0 0) and B-doped (4 × 1016 cm−3) p-type Ge (1 0 0). Following the deposition of a 15 nm SiO2 protection layer, ion implantation processes were carried out with P ions (30 keV, 2 × 1015 cm−2 and 3.2 × 1015 cm−2), Se ions (70 keV, 1 × 1015 cm−2) and Si ions (50 keV, 1 × 1015 cm−2). The implantation energy was selected so that the area of ion implantation could be consumed during later NiGe formation. Tests were
NiGe/Ge contacts doping of p
For samples after germanidation, only mono-phase NiGe was detected by XRD measurements. Fig. 1 shows the current-voltage characteristics of NiGe/n-Ge with phosphorus ion implantation. The reverse current density significantly increases for the NiGe/n-Ge contacts (red curve) in comparison to that of the reference diode with no implanted ions (black curve). The SBH of the NiGe/n-Ge diodes were evaluated by the thermal emission model given by the equation [1], [9]:
Conclusion
As a donor in Ge, phosphorus segregation during germanidation narrows the Schottky barrier of the NiGe/n-Ge contacts, resulting in ohmic characteristics. However, the ohmic-like behavior of Se or Si implanted NiGe/n-Ge contacts should be attributed to the implantation damages near the interface between NiGe and Ge, rather than the segregated Se acting as donors or passivators for the interface states. When the implantation damages are smoothed by thermal annealing, the implanted Se and Si atoms
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
Thanks to Pr.Li Cheng from Xiamen University for his guidance on this article. This work was supported by the National Natural Science Foundation of China under grant Nos. 11705068, 61176092, 61036003, 60837001, the Natural Science Foundation of the Fujian Province under Grants 2017J05011, the financial aid of the National Science-technology Support Plan Projects “Development and application demonstration of temperature monitoring networking platformof third party pharmaceutical cold chain
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